Exhaust gas cleaning system for diesel engines

ABSTRACT

The invention concerns an exhaust gas cleaning system for Diesel engines with a filtration device for separating soot from the exhaust gases and a regeneration device for the filtration device. The filtration device is equipped with filter tubes which are closed at one end. Thus, the exhaust gases can be fed in at the other side and penetrate the filter walls. The soot is deposited on the inner surface of the filter tubes. A burner is provided which generates the burn-off gases for the soot, suitable measures being taken to ensure that the burner gases only act on one section of the filter block at a time. Feed and discharge of the exhaust gases are selected in such a way that the cleaned exhaust gases flow around the filter tubes which are being regenerated. The direction of flow of the exhaust gases can also be reversed so that the exhaust gases first flow around the filter tubes passing through the walls into the tube interiors, and then flow out to the discharge through the openings in the tubes. In this case, the soot is deposited on the external surfaces of the filter tubes.

The invention refers to an exhaust gas cleaning system for Dieselengines.

Filtration of the Diesel exhaust gases is necessary because enginemeasures to reduce soot are not sufficient on their own. A wide varietyof devices fitted with filters has already been suggested for thisafter-treatment of the exhaust gases, using ceramic or electrostaticfilters, for example. These filters are used to filter out the sootparticles from the exhaust gas, which are deposited on the filter. Thisresults in the necessity for removal of the deposited soot from time totime, so as to preserve the functional capacity of the filter. Thisprocess is generally known as filter regeneration.

Thus, SAE Paper No. 850015 describes a device with monolithic ceramicfilters and the regeneration of these filters by specific selection ofmotor setting parameters in conjunction with a filter position close tothe engine. However, in this device filter regeneration is subject tosubstantial random influences, with one of the risks involved being thatthe filter block will become thermally overloaded.

Regeneration can be improved by the metered addition of metallicadditives which are mixed with the fuel, as is described by way ofexample in SAE Publication No. 860137. However, this causes the furtherproblem of the emission of metallic compounds.

A fundamentally different approach to the filter regeneration problemaims at burning off the soot using an additional burner which isswitched on when required and burns away the soot via the burner gases(compare, for example, DE-OS No. 3219948). However, this involves theparticular problem of adapting the burner function to the relevantoperating status, i.e. exhaust gas volume, exhaust gas temperature andexhaust gas pressure of the engine gases. Even small deviations from thesetting required may result in thermal damage to or destruction of thefilter.

In order to avoid these difficulties, change-over devices with twoidentical filters have been suggested, with one filter being located inthe exhaust gas flow at any one time and the other being independentlyregenerated using a burner (DE-OS No. 3204176). The disadvantages ofsuch a device are the large construction volume, the constructionexpense and the required change-over units which are exposed to the hot,corrosive combustion gases.

For this reason, a proposal has also been made for regenerating thefilter sector by sector with an appropriately constructed combustionchamber being passed sector by sector over the filter block (compare,for example U.S. Pat. No. 4,481,767). A major disadvantage here is thefact that because of the variations in heat development and the leveraction of the fixing device, the burner cover is distorted which, inturn, means that the cover cannot seal cleanly against those filtersections which are not being regenerated at any particular time.

In order to avoid this disadvantage, the invention proceeds, fromkinematic reversal of this known proposal, in which the filter isrotated sector by sector into the vicinity of the burner in the knownfashion (U.S. Pat. No. 4,573,317). Apart from its considerableconstruction expense, this generic device has major disadvantages. Theseinclude, in particular, the cooling problem. Since the burner gases andthe soot constitute an additional source of heat, a heat sink withdirect action should be available to protect the components from thermaloverload. Admittedly, a heat sink is present in all the known devices,but this is inadequate so that heat buildup occurs which can veryquickly result in the destruction of the filter.

It is in this respect that the invention should provide a remedy, as theproposed system for the cleaning of exhaust gases no longer has thedisadvantages mentioned above and also produces additional andsubstantial advantages.

The filter elements are advantageously designed in the form of tubes,through the sides of which the exhaust gases flow radially, so that alarge filter area is available. The exhaust gas can be fed to either thefirst or the second segment. Thus, exhaust gases to be cleaned are fedin, passed through the filter tubes and their sides, from one chambersection to the next and in doing so, if the chamber is located in thesecond chamber section, they also pass through those filter tubes withinthe regeneration sector, which are, in fact, disconnected from theexhaust gas cleaning system by the chamber. This means that a heatexchange process occurs between the filter tubes upstream of the chamberand the filtered exhaust gas flow. This process means that the filtertubes of the regeneration sector are either kept at the correcttemperature so that they are already at the same temperature as theexhaust gas when they are pivoted into a filter sector, or the tubes arecooled if the hot burner and soot exhaust gases flow through them whenthey are burned off and overheat them. Thus, the requirement that heatsource and heat sink are spatially adjacent is met here. It is alsoadvantageous that the heat exchange occurs along the entire length ofthe filter tubes.

Advantageously, according to the invention the regeneration burner canalso be switched on when the internal combustion engine is started, sothat the cold combustion exhaust gases are heated by the warmed tubesand the normal smoke emission from the cold engine is prevented. Thisadvantage is not present in the known devices.

It is obvious that designing the filter block in the form of severalspatially separated tubes with a radial flow will prevent the heatbuild-up which may occur with the known monoliths (filter candles). Anadditional advantage here is that the individual filter tubes can bereplaced if necessary. According to the invention, ceramic spiralfilters or steel wool filters can be used as filter elements, eitherdesigned as filter tubes or located within the latter.

According to an advantageous embodiment of the invention, a burner forthe generation of hot burn-off gases generated in the chamber can beused as the regeneration means. However, in principle, instead of this,catalytically active substances can be used for regeneration in thechamber, or provision can be made for chemical oxidizing agents to besprayed in.

According to a further embodiment of the invention, it is particularlyuseful if the filter tubes are bundled about a central shaft in arotationally symmetric fashion and sited in the cylindrical drum, withthe central shaft running on bearings in the housing as the rotationalshaft of the drum.

According to a further embodiment of the invention, the separating wallsin the second segment are arranged radially to the rotational shaftextending outwards to the internal wall of the housing, so that asector-shaped chamber is formed.

According to a useful embodiment of the invention, the filter tubes areconnected to one another on the closed side via a disc-shaped wall ofthe drum and supported by this wall on the shaft so that they cannot berotated. The filter tubes can run on bearings either on one side or onboth sides within the drum. In the event of one-sided fixing, the filtertubes can expand freely and thus reduce tensions; in the case oftwo-sided bearing methods for the filter tubes with a fixed and a freeside, the filter tubes can again expand freely and, in addition, theceramic material is not subjected to shock loads which might affect thehousing. The type of bearing used for the filter tubes is adapted tosuit the individual case.

According to a further advantageous embodiment of the invention, thecleaning capacity can be adapted to the actual residual volumes producedby means of the fact that the cycle frequency is variable for theforward rotation of the filter block as a function of the engine loaddetermined over time.

According to a method in accordance with the invention, it is useful topass the filtered exhaust gases over the regeneration area for coolingor temperature adjustment, so that thermal overload and dew pointproblems at excessively low temperatures are avoided. Regenerationoccurs continuously within the tube-shaped filter element and cleanedcombustion exhaust gas flows continuously around the outside of thefilter element and cools it. The heat exchange process and theregeneration process take place continuously, in parallel and mutuallyindependently. This heat exchange process is thus stationary. Thetemperature field is constant, with all the inherent advantages for theprocess and in terms of material stresses.

Additional characteristics and advantages of the invention can be seenfrom the dependent claims and the following description of embodiments.

The figures show the following.

FIG. 1, a section through the exhaust gas cleaning system in accordancewith the invention;

FIG. 2, a section through Line II--II in FIG. 1;

FIG. 3, a perspective representation of the new exhaust gas cleaningsystem; and

FIGS. 4-6, additional embodiments of the invention.

FIG. 1 shows a preferred embodiment of the invention.

The filter block is located in the cylindrical housing and comprises theindividual filter tubes 2 which are arranged in a rotationally symmetricfashion parallel to the axis, about the central axis 3. The central axis3 runs on bearings as a shaft in the housing 1. In the representationshown in FIG. 1, the left-hand ends of the filter tubes 2 are insertedinto the disc-shaped wall 4 and either sealed at one end by this wall,as shown in the dotted circle, or the tubes 2 penetrate through the wall4 and are integrally closed off. The walls 4 and 5 are appropriatelyconnected with the shaft 3 so that they can be rotated with shaft 3 andthe filter tubes 2 are also rotated with the shaft.

The housing 1 is further subdivided into two mutually sealed sections 6and 7 by the partition 5. The free ends of the filter tubes 2 run onbearings in the partition 5 in such a way that in this case theiropenings are flush with the side of the partition 5 which faces thesection 6, as can be seen clearly in FIG. 3.

Section 6 serves as the admission section for the exhaust gases to becleaned, while filtration of the exhaust gases by the filter tubes 2takes place in section 7. Section 6 is further subdivided by partitions8 and 9 which extend radially from the shaft 3 to the internal wall ofthe housing 1 and which are fixed. A sector-shaped chamber 10 is thusobtained which is sealed off from the remaining space and isconsiderably smaller than the rest of the filtration area. The burner 11projects into the sector-shaped chamber 10 so that it can be describedas the regeneration area. FIG. 2 shows the design of the regenerationarea. The exhaust gases to be cleaned are fed in via the connectionpiece 12 and the cleaned exhaust gases are carried off through theconnection piece 13. The filter block which consists of the individualfilter tubes and walls 4 and 5 can be rotated in the housing 1 by meansof the motor 14 which is shown schematically.

The mode of operation of the exhaust gas cleaning system will bedescribed below.

It is assumed that the filter block is in the position shown in FIG. 1.The exhaust gas flows in the direction of the arrow 15 through theconnection piece 12 into the space separated from the regeneration areaand thence through the openings 16 of the filter tubes 2. Since thefilter tubes 2 are sealed off at the opposite end, the exhaust gases areforced to penetrate the filter walls and this occurs along the entirelength of the filter tubes, so that large effective filter areas areobtained. As the gases flow through the tubes 2, the suspendedparticles, i.e., the soot, are deposited on the internal walls of thefilter tubes. On their way to the outflow connection piece 13, thecleaned exhaust gases flow around the filter tubes in the regenerationarea, thus exerting a cooling action, and combine with the regenerationgases before flowing out in the direction of arrow 17. No exhaust gasescan reach the filter tubes 2 in the regeneration section 10 because ofthe seal formed by partitions 8 and 9. Instead, the burner gases enterthese filter tubes and are thus heated up to the ignition point of thedeposited soot so that the soot ignites and burns off. It may beadvisable to switch off the burner after this burning-off process hascommenced.

After regeneration of the relevant filter tubes, the filter block isrotated by one sector width with the motor 14. Of course, it is alsopossible to rotate the filter block continuously instead of using cycleoperation. Seals 18 are located between the filter block and housing toprevent uncleaned exhaust gases entering the regeneration area. The sameapplies for the slots between the fixed partitions 8 and 9 and themovable filter block, for which a strip seal is provided, which is notdescribed in any greater detail. The external face of partition 5 slidesover this strip seal. In order to reduce wear between the seals and thepartition 5, it is useful to remove the load from the seals duringrotation by means of slight axial displacement. This can be achieved byaxial displacement of the shaft 3, for example, using a suitable meanswhich is not described in any greater detail. Lifting can also, however,be kept so slight or omitted altogether that the rotating surfaces rubagainst one another, so that the soot on the outsides of the filtertubes is scraped off and these surfaces are also cleaned. In the lattercase, the seals 18 described above must be suitably resistant.

In the example described, the filter tubes are cleaned in cyclicfashion; it is possible to determine the cycle time, i.e. theregeneration time, and make it dependent on the amount of soot depositedon the filter tubes, for example. To this end, the deposit conditionmust be monitored and measured; this can be accomplished using pressuremeasurements, where the pressure difference between the static pressuresupstream and downstream of the regeneration segment is calculated. FIG.1 indicates sensors 19 and 20 which can be used for this purpose indiagrammatic form. The pressure difference can be measured when theburner has just been started (hot method) or when the burner air hasjust been started (cold method). The differential pressure measurementis all the more reliable because a defined burner-air flow is presentand it is not necessary to depend on the varying operating statuses ofthe engine exhaust gases.

Another measure, quite well-known, can also be used with the new systemfor the cleaning of exhaust gases, namely coating with catalyticallyactive substances. It has the advantage of complete combustion of thehydrocarbons absorbed in the soot. This is necessary, in particular, ifthe desorption of the hydrocarbons by heat transmission occurs morerapidly than the burn-off process, i.e., the exhaust gases would nototherwise be sufficiently combusted. In this context, it is useful andadvisable for the catalytic coating to be provided on the discharge sideof the filters.

It is easy to see that a wide variety of alterations and supplements tothe described exhaust gas cleaning system are left to the expert,without departing from the scope of the invention.

One alteration which is useful for certain applications involvesreversal of the direction of flow of the exhaust gases, i.e., theexhaust gases are fed in to the connection piece 13 and discharged fromthe connection piece 12 after having been cleaned. In this case, thecombustion exhaust gases flow around the filter tubes 2 and penetratetheir walls, so that the soot is deposited on the external surfaces ofthe filter tubes 2.

This reversal means that the deposit of residual particles and theadmission of the burner gases now occur as an opposed flow, in contrastto FIG. 1, where these two processes occur in a parallel flow.

FIG. 4 shows an embodiment of this type, where the only difference fromFIG. 1 is that the inlet connection piece 21 is staggered in relation tothe discharge connection piece 13. Otherwise, this system works inprecisely the same way as the system described in accordance with FIG.1, so that further explanation is not necessary.

On the other hand, it is possible to reverse the direction of flow ofthe exhaust gases again in the example in FIG. 4. In this case, it isnecessary for the left-hand ends of the filter tubes 2 to be open andthe right-hand ends of the filter tubes 2 to be closed, as can be seenfrom the flow arrows in FIG. 5 which will not be described in anyfurther detail. Thus, here, too, soot deposit also occurs on theexternal surfaces of the filter tubes 2, but so is the supply of the hotburner gases, so that a parallel flow is formed again. In this example,in order to separate the regeneration area from the rest of the filterblock, radial partitions are present in the filter block, separating itinto sections whose cross section is identical to that of the combustionchamber.

It is also possible to locate the burner 11 otherwise than shown inFIGS. 1 to 5. FIG. 6, which corresponds to FIG. 4, shows an alterationof this type where the burner 22 acts at the circumference of the filterblock. Here, too, partitions are required in the filter block asindicated in the previous example. Once again, deposit of the residualparticles and admission of the hot burner gases occur in a parallelflow. A detailed description is not necessary since the mode of actionof this example corresponds to the embodiment described in the previousparagraph.

All the characteristics described in the description, the claims belowand the drawing may constitute the substance of the invention, eitherindividually or in any combination.

I claim:
 1. An exhaust gas cleaning system for Diesel engines includinga filtration device for separating soot from the exhaust gases and aregeneration device for the filtration device, comprising a separateregeneration sector with the admission of regeneration means for thesoot; said filtration device comprises a number of filter elements,preferably arranged equidistantly, the filter elements being located ina rotatable drum which can be pivoted on bearings within a housing whichis sealed to prevent leakage of the exhaust gases and has an admissionand discharge line; one wall of said drum subdivides the housing intotwo parts with holes to allow axial throughflow of the exhaust flowbetween the first section containing the filter elements and a secondsection towards which the filter elements are open ended and in whichregeneration takes place relative to said filter elements, depending onthe rotational position of the drum, in one single corresponding sectionof the filtration device; said filter elements (2) are tube-shaped andclosed off at one end by a wall (4) such that the exhaust gases flowradially through the filter tube (2) and axially through a perforatedwall (5), and include partitions (8 and 9) which run parallel to thefilter tubes and are present between the perforated wall (5) of the drumand the housing wall and situated relative to the walls (4 and 5) of thedrum, forming a chamber (10) in at least one of the sections (6, 7) forthe regeneration means.
 2. An exhaust gas cleaning system for Dieselengines in accordance with claim 1, in which a burner for the generationof hot burn-off gases is used as the regeneration means, the gases beinggenerated in the chamber (10).
 3. An exhaust gas cleaning system forDiesel engines in accordance with claim 1 in which said filter elements(2) are located in the drum in a rotationally symmetric fashion parallelto the axis and about a shaft (3) and that the shaft (3) is connectedflush with the drum.
 4. An exhaust cleaning system for Diesel engines inaccordance with claim 3, in which said partitions (8 and 9) in thesecond section (6) extend radially from the shaft (3) in a fixedposition to an internal wall of the housing and are fixed with respectto this wall so that a sector-shaped combustion chamber (10) is formed.5. An exhaust gas cleaning system for Diesel engines in accordance withclaim 4, in which said combustion chamber (10) is in a fixed positionand the drum with the filter elements can be rotated, particularly in astepwise fashion, with the steps being the size of a sector upstream ofthe combustion chamber (10).
 6. An exhaust gas cleaning system forDiesel engines in accordance with claim 5, in which a cycle frequencyfor further drum (filter block) rotation can be varied as a function ofthe engine load determined over time.
 7. An exhaust gas cleaning systemfor Diesel engines in accordance with claim 2, in which the axis of theburner (11) is parallel to the axes of the filter elements (2).
 8. Anexhaust gas cleaning system for Diesel engines in accordance with claim4, which includes a connection piece (13) for discharging the filteredexhaust gases and that the axis of this connection piece (13) liesroughly along the axis of symmetry of the sector-shaped combustionchamber and the center plane of the first section (7).
 9. An exhaust gascleaning system for Diesel engines in accordance with claim 1, in whichthe closed end of the filter elements (2) are interconnected via adisc-shaped drum wall (4) and supported on the shaft (3) by this wall(4) in a manner fixed against rotation relative to each other.
 10. Anexhaust gas cleaning system for Diesel engines in accordance with claim1, which includes a control means for switching on the regenerationprocess by a decrease in pressure of the exhaust gas in the sector to beregenerated.
 11. An exhaust gas cleaning system for Diesel engines inaccordance with claim 1, in which said filter elements (2) are coatedwith catalytically active substances on the external surface on the flowside.
 12. An exhaust gas cleaning system for Diesel engines inaccordance with claim 1, in which seals (18) are provided between thehousing (1) and the front wall (5) and that when the drum (filter block)is operated with cyclic rotation the front wall (5) lifts away from theseals (18) for the rotation operation.
 13. An exhaust gas cleaningsystem for Diesel engines in accordance with claim 1, in which thedirection of flow of the exhaust gases is reversible.
 14. An exhaust gascleaning system for Diesel engines in accordance with claim 2, in whichsaid burner (22) is located on the housing (1) in such a way that theflame direction is perpendicular to the filter element (2) (FIG. 6). 15.An exhaust gas cleaning system for Diesel engines in accordance withclaim 2, in which said filter elements (2) are located in the drum in arotationally symmetric fashion parallel to the axis and about a shaft(3) and that the shaft (3) is connected flush with the drum.
 16. Anexhaust gas cleaning system for Diesel engines in accordance with claim3, in which the axis of the burner (11) is parallel to the axes of thefilter elements (2).
 17. An exhaust gas cleaning system for Dieselengines in accordance with claim 5, which includes a connection piece(13) for discharging the filtered exhaust gases and that the axis ofthis connection piece (13) lies roughly along the axis of symmetry ofthe sector-shaped combustion chamber and the center plane of the firstsection (7).
 18. An exhaust gas cleaning system for Diesel engines inaccordance with claim 6, which includes a connection piece (13) fordischarging the filtered exhaust gases and that the axis of thisconnection piece (13) lies roughly along the axis of symmetry of thesector-shaped combustion chamber and the center plane of the firstsection.
 19. An exhaust gas cleaning system for Diesel engines inaccordance with claim 2, in which the closed end of the filter elements(2) are interconnected via a disc-shaped drum wall (4) and supported onthe shaft (3) by this wall (4) in a manner fixed against rotationrelative to each other.
 20. An exhaust gas cleaning system for Dieselengines in accordance with claim 2, which includes a control means forswitching on the regeneration process by a decrease in pressure of theexhaust gas in the sector to be regenerated.
 21. An exhaust gas cleaningsystem for Diesel engines in accordance with claim 2, in which saidfilter elements (2) are coated with catalytically active substances onthe external surface on the flow side.
 22. An exhaust gas cleaningsystem for Diesel engines in accordance with claim 2, in which seals(18) are provided between the housing (1) and the front wall (5) andthat when the drum (filter block) is operated with cyclic rotation thefront wall (5) lifts away from the seals (18) for the rotationoperation.
 23. A method for cleaning exhaust gases of a Diesel enginewhich comprises directing the exhaust gases into one end of a closedhousing containing a plurality of filter tubes in a closed portion ofsaid housing adjacent said closed end with each of said tubes having oneend closed, directing the exhaust gases through said filter tubes, tocollect soot on an inner surface thereof, rotating said plurality offilter tubes thereby positioning said filter tubes in a vicinity of aself-contained regeneration means in said one end of said closedhousing, regenerating said filter tubes in the vicinity of saidregeneration means, and rotating said plurality of filter tubes therebymoving other filter tubes to the vicinity of said regenerating means toregenerate said filter tubes and successively rotating said filter tubesto said regeneration means until all filter tubes have been regenerated.24. A method for the cleaning of Diesel exhaust gases in accordance withclaim 23, in which the exhaust gases which have already been filtered toremove soot particles can be used for a heat exchange process in aregeneration area operating by thermal means.